(1) Field of the Ivention
The present invention relates to the field of rotorcraft, and it relates more specifically to rotorcraft anti-torque rotors of rotary drive axis that is substantially horizontal. Typically, such an anti-torque rotor serves to stabilize and guide the rotorcraft in yaw, by countering the yaw torque generated by the main rotor of rotary drive axis that is substantially vertical and that serves to provide the rotorcraft at least with lift.
(2) Description of Related Art
The anti-torque rotor of the present invention is installed more particularly at the end of a tail boom of the rotorcraft. The rotor disk constituted by the rotary wing of said anti-torque rotor is oriented mainly vertically and longitudinally, being located laterally, i.e. on one side, of the tail boom of the rotorcraft, such that the anti-torque rotor generates a transverse thrust vector component that, in flight, provides yaw control of the rotorcraft.
The concepts of “lateral”, or in other words “on one side”, “transverse”, and “vertical” are concepts that are commonly understood in the field of rotorcraft, being relative to the concept of “longitudinal”, which is defined along the general direction in which the rotorcraft extends on the ground, which is typically considered as extending longitudinally from front to rear.
In general terms, rotorcraft rotors typically comprise a rotary wing made up of blades that are radially distributed around a hub. The hub is driven in rotation by a mechanical power transmission gearbox that is engaged with a power plant of the rotorcraft. While it is being driven in rotation, the rotary wing conventionally defines a rotor disk that extends between the tips of the blades driven in rotation by the hub.
With rotors, a distinction is typically drawn between the rotary drive axis of the rotor and the geometrical axis of rotation of the rotor. The rotary drive axis of the rotor is identified by the axis of rotation of the hub carrying the rotary wing, whereas the axis of rotation of the rotor corresponds to the geometrical axis of rotation of the rotor disk formed by the rotary wing of the rotor.
The blades are individually mounted on the hub via respective blade roots. The blade roots may be incorporated in the blades or they may be formed by mounting arms having the blades fitted thereto. Such a mounting arm may for example be arranged as a cuff or a sleeve.
A pilot of the rotorcraft can cause the blades of a rotorcraft rotor to pivot about respective pitch variation axes oriented along the general direction in which each blade extends. The blades are caused to pivot about their pitch variation axes by means of a blade control mechanism that can be actuated by a linkage operated by the pilot generating flight commands. Said pilot may be a human pilot or an autopilot.
In order to cause the blades to pivot about their pitch variation axes, each blade root is individually mounted to pivot on the hub, at least about the pitch variation axis of the blade. Each blade root has a pitch lever for individually engaging said control mechanism via a respective control link.
These arrangements are such that the pilot can vary the angle of incidence of the blades of the various rotors of a rotorcraft in order to modify the propulsion and/or the attitude in flight of the rotorcraft along the various directions in which it extends including the longitudinally-extending direction, the transversely-extending direction, and/or the vertically-extending direction.
Conventionally, rotorcraft have at least one main rotor with a rotary drive axis that is substantially vertical for the purpose of providing the rotorcraft at least with lift and/or with guidance in the vertically-extending direction of the rotorcraft.
In the specific configuration of helicopters, the main rotor not only provides the rotorcraft with lift, but also with propulsion in any direction of progression, and it enables the rotorcraft to change attitude in pitching and in roll.
For this purpose, the blades of the main rotor are movable by the pilot so as to pivot about their pitch variation axes. In order to modify the lift provided by the main rotor, the pilot generates flight commands that cause the pitch of the blades of the main rotor to vary collectively. In order to modify the attitude of the rotorcraft in pitching and/or in roll, the pilot generates flight commands that cause the pitch of the blades of the main rotor to vary cyclically.
With the main rotor, said mechanism for controlling the pitch of the blades about their pitch variation axes frequently comprises a swashplate mounted on a mast carrying the main rotor on its drive axis. The swashplate is made up of a bottom plate carrying a top plate that is superposed thereon and that lies on the same axis. The bottom plate is mounted to be stationary in rotation about the drive axis of the main rotor. The top plate is mounted to rotate about the drive axis of the main rotor, being put into engagement with the hub by means of a hinged structure, e.g. arranged as a scissors link.
Furthermore, the bottom plate is mounted to move relative to the mast in translation and in nutation. The bottom plate can be moved by the pilot by means of control links operating in response to three distinct flight control lines. The top plate is connected to each of the blade roots by control links respectively engaged with the pitch levers of each of the blade roots. Such an arrangement of the swashplate enables it to be moved axially while also being capable of oscillating in all directions like a ball joint, so as to cause the pitch of the blades to vary in compliance with the flight commands issued by the pilot.
These arrangements mean that the pilot can place the swashplate in any orientation in three dimensions relative to the mast. A movement of the swashplate in translation along the mast causes the pitch of the blades to vary collectively and serves to modify the lift produced by the main rotor, thus making it possible to vary the flight attitude of the rotorcraft in its vertically-extending direction. Tilting of the swashplate relative to the mast depending on the individual azimuth positions of the blades causes the pitch of the blades to vary cyclically and thus enables the flight attitude of the rotorcraft to be modified in pitching and/or in roll.
Proposals also have been made in Document US 2011/211953 (Brandon L. Stille) for a mechanism analogous to two plates for varying the pitch of the blades of a rotorcraft rotor. According to that document, the outer plate engaging the blades is arranged around an inner plate that is mounted to oscillate in its general plane. The two plates can be moved together in translation by a main rod for varying the pitch of the blades collectively. A secondary rod coaxial with the main rod enables the inner plate to be inclined and consequently enables the outer plate to be inclined relative to the axis of the rotor in order to vary the pitch of the blades cyclically.
Furthermore, rotorcraft are conventionally fitted with an anti-torque device providing the rotorcraft with stability in yaw by countering the yaw torque generated by the main rotor. Such an anti-torque device is also used for guiding the rotorcraft in yaw. Anti-torque devices of rotorcraft are frequently installed at the end of a tail boom of the rotorcraft. By way of example, an anti-torque device for a rotorcraft may be of the air-jet type, or more commonly it may be formed by a tail rotor having a rotary drive axis that is substantially horizontal.
With such a tail rotor, the rotor disk formed by the tail rotary wing is oriented mainly vertically and longitudinally, and in particular is arranged on one side of the tail boom of the rotorcraft. These arrangements are such that the tail rotor generates thrust that mainly comprises a transverse vector component for providing the rotorcraft with yaw control.
The stabilization and the guidance of the rotorcraft in yaw are controlled by causing the pitch of the blades of the tail rotor to vary collectively, thereby varying the magnitude of the thrust generated by the tail rotor. For this purpose, the tail rotor is fitted with a said control mechanism for varying the pitch of its blades about their pitch variation axes.
In a conventional embodiment, such a control mechanism for a tail rotor comprises a control rod mounted to move in translation relative to a structure for mounting the tail rotor on the tail boom. Such a structure is formed in particular by a mechanical power transmission gearbox that provides an angle takeoff to connect the tail rotor to a drive shaft extending orthogonally relative to the drive axis of the tail rotor.
The control rod extends inside the hub and it is mounted to be stationary in rotation. The control rod is movable in translation by means of a control link operated by the pilot using a control linkage for controlling the attitude of the rotorcraft in yaw.
A control plate is rotatably mounted on the control rod and it carries operating links engaged with respective levers fitted individually to the blade roots carrying the blades of the tail rotor. These arrangements are such that the pilot causing the control rod to move in translation leads to the pitch of the blades of the tail rotor being varied collectively.
Furthermore, the forces for pivotally moving the blades of the various rotors of a rotorcraft can be considerable, so it can be useful for the pilot to be assisted in delivering the forces for controlling the blades. For this purpose, it is common practice to use servo-controls placed on the various control linkages in order to cause the pitch of the blades in the various rotors to vary.
More particularly, servo-controls may provide a human pilot with assistance in delivering the forces that need to be delivered in order to vary the pitch of the blades by means of a power transmission mechanism. The servo-controls may also advantageously be controlled as a function of flight commands generated by an autopilot.
In this context, it has been found that use of the tail rotor can be optimized by using thrust from the tail rotor not only for stabilizing and guiding the rotorcraft in yaw, but also for contributing to providing it with propulsion in translation. More particularly, the tail rotor can be used not only for controlling the attitude of the rotorcraft in yaw, but also for forming a propeller for propelling the rotorcraft in translation.
Nevertheless, in order to provide such a propeller for propulsion in translation, the rotor disk formed by the rotary wing of the tail rotor needs to be oriented mainly vertically while being inclined relative to the orientation of the transversely-extending plane of the rotorcraft.
One known solution consists in swivel-mounting the tail rotor on the tail boom, such that the rotor disk can be oriented in various directions depending on the use that is being made of the tail rotor.
More particularly, the tail rotor may be swiveled between a position in which the rotor disk is in a longitudinal-vertical orientation and a position in which the rotor disk is in a transverse-vertical orientation.
In the longitudinal-vertical orientation position, the rotor disk is oriented vertically and longitudinally in the directions in which the rotorcraft extends vertically and longitudinally. In other words, in the longitudinal-vertical orientation position, the rotor disk is arranged substantially perpendicularly to the transversely-extending direction of the rotorcraft.
In the transverse-vertical orientation position, the rotor disk is oriented vertically, being at least inclined relative to the longitudinally-extending direction of the rotorcraft, or indeed being arranged perpendicularly relative thereto.
Those arrangements are such that when the rotor disk is positioned in the longitudinal-vertical orientation, the tail rotor is used solely for guiding and stabilizing the rotorcraft in yaw against the yaw torque generated by the main rotor. Swiveling the tail rotor so as to position the rotor disk in its transverse-vertical orientation then enables the thrust produced by the tail rotor to be used to contribute to propelling the rotorcraft in translation.
On this topic, reference may be made to Document FR 2 969 577 (Eurocopter), which describes such techniques for swiveling a tail rotor so that the rotor disk is steered selectively between a longitudinal-vertical orientation and a transverse-vertical orientation on either side of a neutral orientation.
Another known solution consists in permanently orienting the rotor disk formed by the rotary wing of the tail rotor in a longitudinal-vertical position that is more specifically oriented orthogonally relative to the longitudinally-extending direction of the rotorcraft, and then to cause the pitch of the blades to vary collectively and/or cyclically depending on requirements. On this topic, reference may be made to Document GB 622 837 (Firestone Tire & Rubber Co.) or to Document FR 1 484 732 (Dornier Werke Gmbh), which describe such ways of operating a tail rotor.
According to Document FR 1 484 732, the pitch of the blades is varied by operating a swashplate in the same manner as the swashplate conventionally used for varying the pitch of the blades of a main rotor. Collective variation of the pitch of the blades serves to vary the amplitude of the thrust produced by the tail rotor and thus makes it possible to adjust rotorcraft thrust in translation by means of the tail rotor. Stabilization and guidance of the rotorcraft are obtained by cyclical variation of the pitch of the blades of the tail rotor in association with making use of a rudder.
According to Document GB 622 837, a two-plate mechanism is mounted on a bushing surrounding the axis of rotation of a tail rotor. The two-plate mechanism comprises a rotary outer plate placed around an inner plate that does not rotate. The outer plate is engaged with the blades via a linkage for varying pitch by moving the outer plate axially. The inner plate is mounted to oscillate so that its angle of inclination, and consequently the angle of inclination of the outer plate, leads to cyclical variation in the pitch of the blades.
Another problem posed by rotorcraft rotors lies in flapping movements of the blades in the general plane of the rotor disk formed by the rotary wing.
For a tail rotor, reference may be made on this topic to Document GB 2 274 634 (Westland Helicopters), which proposes countering such flapping movements of the blades of a tail rotor by causing their pitch to vary cyclically.
According to GB 2 274 634, the control rod has a plate for controlling the blades in pivoting about their pitch variation axes. The control rod is mounted to turn together with the rotary wing on a tail rotor mounting structure at the end of a tail boom of the rotorcraft, being movable in translation along the drive axis of the tail rotor. In addition, the control rod has a ball joint hinge and is movable in nutation by means of an actuator in order to cause the control plate to be inclined and thereby in order to cause the pitch of the blades to vary cyclically on each rotation of the tail rotor.
Another use that is known for a tail rotor of a rotorcraft lies in providing the main rotor with assistance in providing lift. For this purpose, a tail rotor is provided at the end of the tail boom of a rotorcraft in such a manner that the rotor disk formed by its rotary wing is arranged in a longitudinal-sloping orientation. Such a longitudinal-sloping orientation is given to the rotor disk by mounting the tail rotor on the rotorcraft so that its drive axis is arranged in a manner that slopes significantly relative to the horizontally-extending plane of the rotorcraft.
In the longitudinal-sloping orientation position of the rotor disk, the tail rotor serves not only mainly to provide stabilization and guidance of the rotorcraft in yaw by means of a transverse thrust vector component, but also to provide additional lift by means of a vertical thrust vector component, making it possible to increase the range over which the center of gravity of the rotorcraft can be extended rearwards. The additional lift provided by the tail rotor is advantageous under specific flight situations, such as when transporting heavy loads and/or when the aircraft is hovering or flying at low speeds, which are commonly identified as being speeds less than 50 knots (kt).
Nevertheless, it has been found in practice that such additional lift can be harmful under certain flight situations of the rotorcraft, such as in particular when the rotorcraft is in a stage of flight at speeds faster than 75 kt.
In cruising flight, additional lift provided by the tail rotor provides an unfortunate increase in the attitude hump (effect of rotor wash on the stabilizer of the rotorcraft), degrades the stability of the rotorcraft, and leads to excessive fuel consumption. Consequently, continuous use of the tail rotor to provide additional lift is not appropriate, particularly when the rotorcraft is flying at cruising speeds, or indeed when the rotorcraft is not heavily loaded.
It can thus be seen that there has been continuous research in the field of rotorcraft concerning the organization of a tail rotor to provide not only control over the attitude of the rotorcraft in yaw, but also enabling the rotorcraft to be propelled in other directions, such as along the gravity axis in order to provide the rotorcraft with additional lift or along other directions so as to enable the rotorcraft to progress in translation.
Such research involves making choices concerning the uses to be made of the thrust generated by the tail rotor in order to provide on a priority or a subsidiary basis transverse thrust, vertical thrust, or horizontal thrust.
Nevertheless, a compromise needs to be found in such research between optimized use of the thrust provided by the tail rotor and simplicity in the structure of the tail rotor. It is important to avoid excessively complexifying the organization of the tail rotor, given that the advantages obtained are marginal compared with priority use of the tail rotor for controlling the attitude of the rotorcraft in yaw.